Internet Engineering Task Force R. Despres, Ed.
Internet-Draft RD-IPtech
Intended status: Experimental B. Carpenter
Expires: October 25, 2012 Univ. of Auckland
D. Wing
Cisco
S. Jiang
Huawei
April 23, 2012
Native IPv6 Behind NAT44 CPEs (6a44)draft-despres-6a44-01
Abstract
In customer sites having IPv4-only CPEs, Teredo provides a last
resort IPv6 connectivity [RFC4380] [RFC5991] [RFC6081]. However,
because it is designed to work without involvement of Internet
service providers, it has significant limitations (connectivity
between IPv6 native addresses and Teredo addresses is uncertain;
connectivity between Teredo addresses fails for some combinations of
NAT types). 6a44 is a complementary solution that, being base on ISP
cooperation, avoids these limitations. 6a44 uses specific prefixes
assigned by local ISPs (rather than the anycast address used by
Teredo, an evolution similar to that from 6to4 to 6rd). The
specification is complete enough for actual deployment, including
with independently written codes.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 25, 2012.
Copyright Notice
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Internet-Draft Native IPv6 Behind NAT44 CPEs (6a44) April 20121. Introduction
Although most customer premise equipments (CPE's) should soon be
dual-stack capable, a large installed base of IPv4-only CPE's is
likely to remain for several years. Their operation is based on IPv4
NAT's (NAT44's). Also, due to the IPv4 address shortage, more and
more Internet service providers (ISP's), and more and more mobile
operators, will assign private IPv4 addresses of [RFC1918] to their
customers (the [NAT444] model). For a rapid and extensive use of
IPv6 [RFC2460], there is therefore a need for IPv6 connectivity
behind NAT44's, including those of the [NAT444] model.
At the moment, there are two tunneling techniques specified for IPv6
connectivity behind NAT44's:
o Configured tunnels. They involve tunnel brokers with which users
must register [RFC3053]. Well-known examples include deployments
of the Hexago tool, and the SixXs collaboration, which are
suitable for IPv6 early trials. However, this approach is not
adequate for mass deployment: it imposes that, even if two hosts
are in the same customer site, IPv6 packets between them must
transit via tunnel servers, which may be far away.
o Automatic Teredo tunnels [RFC4380] [RFC5991]. Teredo is specified
as a last resort solution which, due to its objective to work
without local ISP involvement, has the following limitations:
* Connectivity between IPv6 native addresses and Teredo addresses
is uncertain. (As explained in [RFC4380] section 8.3, this
connectivity depends on paths being available from all IPv6
native addresses to some Teredo Relays. ISP's lack sufficient
motivations to ensure it).
* Between two Teredo addresses, IPv6 connectivity fails for some
combinations of NAT44 types([RFC6081] section 3).
* According to [RFC4380] section 5.2, each Teredo host has to be
configured with the IPv4 address of a Teredo server (a
constraint that can however be avoided in some
implementations).
6a44 is designed to avoid Teredo limitations where ISP's can
participate to the solution. The approach for this is similar to
that which permitted 6rd [RFC5569] [RFC5969] to avoid limitations of
6to4 [RFC3056] [RFC3068]: at the beginning of IPv6 addresses, the
Teredo well-known prefix is replaced by network specific prefixes
assigned by local ISP's.
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This document is organized as follows: terms used in the document are
defined in Section 3; design goals and model of operation are
presented in Section 4; Section 5 describes the format of 6a44 IPv6
addresses; Section 6 specifies in details behaviors of 6a44 clients
and 6a44 relays; security and IANA considerations are respectively
covered in Section 7 and Section 8.
The specification is expected to be complete enough for running codes
to be independently written and the solution to be incrementally
deployed and used. Its intended status is Experimental rather than
Standard to reflect uncertainty as to which major Internet players
may be willing to support it.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Definitions
The following definitions are used in this document:
MAJOR NEW DEFINITIONS
"6a44 ISP network": An IPv4-capable ISP network that supports at
least one 6a44 relay. Additional conditions are that it assigns
individual IPv4 addresses to its customer sites (global or
private), that it supports ingress filtering [RFC2827], and that
its path MTU's are at least 1308 octets.
"6a44 relay": A node that supports the 6a44 relay function defined
in this document, and that has interfaces to an IPv6-capable
upstream network and to an an IPv4-capable downstream network.
"6a44 client": A host that supports the 6a44 client function defined
in this document, and has no other mean than 6a44 to have a IPv6
native address.
"6a44 tunnel": A tunnel established and maintained between a 6a44
client and 6a44 relays of its ISP network.
"6a44 bubble": A UDP/IPv4 packet sent from a 6a44 client to the
6a44-relay address, or conversely, and having a UDP payload that
cannot be confused with an IPv6 packet. In the client to relay
direction, it is a request for a response bubble. In the relay to
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client direction, it conveys the up-to-date IPv6 prefix of the
client.
SECONDARY NEW DEFINITIONS
(for reference, can be skipped by readers familiar with usual
terminology)
"6a44 service": The service offered by a 6a44 ISP network to its
6a44 clients.
"6a44-client IPv6 address": The IPv6 address of a 6a44 client. It
is composed of the client IPv6 prefix, received from a 6a44 relay,
followed by the client local IPv4 address.
"6a44-client IPv6 prefix": For a 6a44 client, the IPv6 prefix (/96)
composed of the IPv6 prefix of the local 6a44-network (/48)
followed by the UDP/IPv4 mapped address of the client (32 + 16
bits).
"6a44-client UDP/IPv4 mapped address": For a 6a44 client, the
external UDP/IPv4 address that, in the CPE NAT44 of the site, is
that of its 6a44 tunnel.
"6a44-client UDP/IPv4 local address": For a 6a44 client, the
combination of its local IPv4 address and the 6a44 port.
"6a44 port": The UDP port used for 6a44 (see Section 8).
"6a44-relay UDP/IPv4 address": The UDP/IPv4 address composed of the
6a44-relay anycast address and the 6a44 port.
"6a44-relay anycast address": The well-known IPv4 anycast address of
6a44 relays (see Section 8).
"6a44-network IPv6 prefix": An IPv6 /48 prefix assigned by an ISP to
a 6a44 network.
USUAL DEFINITIONS
"Upstream direction": For a network border node, the direction
toward the Internet core.
"Downstream direction": For a network border node, the direction
toward end-user nodes (opposite to the upstream direction).
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"IPv4 private address": An address that starts with one of the three
prefixes of [RFC1918] (10/8, 172.16/12, or 192.168/16).
"IPv6 native address": An IPv6 global unicast address that starts
with an aggregetable prefix assigned to an ISP.
"UDP/IPv4 address": The combination of an IPv4 address and a UDP
port.
"UDP/IPv4 packet": A UDP datagram contained in an IPv4 packet.
"IPv6/UDP/IPv4 packet": An IPv6 packet contained in a UDP/IPv4
packet.
4. Design Goals, Requirements, and Model of Operation4.1. Hypotheses about NAT Behavior
Although 6a44 will not work with all possible NAT44 behaviors, 6a44
is designed to work with endpoint-dependent NAT44 mappings as well as
with endpoint-independent mappings, including if there are dynamic
changes from one mode to the other.
The only assumption is that, after a mapping has been established in
the NAT44, it is maintained as long as it is re-used at least once,
in each direction, every 30 seconds.
NOTE: 30 seconds is the value used for the same mapping-maintenance
purpose in Teredo [RFC4380], and in SIP [RFC5626].
4.2. Native IPv6 Connectivity for unmanaged Hosts behind NAT44's
The objective remains that, as soon as possible, CPEs and ISPs
support IPv6 native prefixes. 6a44 is therefore designed only as a
temporary solution for hosts to obtain IPv6 native addresses in sites
whose CPEs are not IPv6-capable yet.
As noted in Section 1, IPv6 native addresses obtainable with
configured tunnels have important limitations. However, compared to
6a44 addresses, they have the advantage of remaining unchanged in
case of NAT44 reset. 6a44 remains therefore the last resort solution
for IPv6 native addresses in unmanaged hosts of IPv4-only-CPE sites,
while configured tunnels may still be preferred for some managed
hosts if reported limitations of configured tunnels are consciously
found acceptable. Their scopes being different, the two solutions
can usefully coexist.
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Note that Teredo remains a last resort solution for hosts to have
IPv6 addresses where IPv6 native addresses cannot be available (and
where Teredo limitations are consciously found acceptable).
4.3. Operational Requirements
Operational requirements of 6a44 include the following:
"Robust IPv6 connectivity": A node having a 6a44 address must have
paths across the Internet to and from all IPv6 native addresses
that are not subject to voluntary firewall filtering.
"Intra-site path efficiency": Packets exchanged between 6a44 clients
that are behind the same CPE NAT44 must not have to traverse it.
If these clients have IPv4 connectivity using their private IPv4
addresses, they must also have IPv6 connectivity using their 6a44
addresses.
"Plug-and-play operation of 6a44 clients": In order to obtain a 6a44
address from its local ISP, a 6a44 client must need no parameter
configuration.
"Scalability of ISP functions": For the solution to be easily
scalable, ISP-supported functions have to be completely stateless.
"Anti-spoofing Protection": Where address anti-spoofing is ensured
in IPv4 with ingress filtering of [RFC2827] [RFC3704], IPv6
addresses must benefit from the same degree of anti-spoofing
protection.
"Overall Design simplicity": As Antoine de Saint-Exupery said in
[The Tool], "it seems that perfection is attained not when there
is nothing more to add, but when there is nothing more to remove".
"Incremental deployability": Hosts and ISP networks must be able to
become 6a44 capable independently of each other. IPv6 must be
operational where both are available, and there must be no
perceptible effect where they are not both available.
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Figure 2 illustrates paths of IPv6 packets in between a 6a44 client H
and various possible locations of remote hosts (A in the same site, B
in another 6a44 site of the same ISP, C in a non-6a44 IPv6 site of
the same ISP, D in an IPv6 site of another ISP). Between 6a44
clients of a same site, IPv6 packets are encapsulated in IPv4
packets. Those Between 6a44 clients and 6a44 relays are encapsulated
in UDP/IPv4 packets.
6a44 operates as follows (details in Section 6):
1. A 6a44 client starts operation by sending a 6a44 bubble to the
6a44-relay UDP/IPv4 address.
2. When a 6a44 relay receives a bubble from one of its 6a44
clients, it returns to this client a bubble containing the IPv6
prefix of this client.
3. When a 6a44 client receives a bubble from a 6a44 relay, it
updates (or confirms) its 6a44 address. It is an update if the
client has no IPv6 address yet or if, due to a CPE reset, this
address has changed. After receiving a bubble, a client is
ready to start, or to continue, IPv6 operation.
4. When a 6a44 client having a 6a44 address has an IPv6 packet to
send whose destination IS in the same customer site, it
encapsulates it in an IPv4 packet whose destination is found in
the IPv6 destination address. It then sends the resulting IPv6/
IPv4 packet.
5. When a 6a44 client receives a valid IPv6/IPv4 packet from a 6a44
client of the same site, it decapsulates the IPv6 packet and
submits it to further IPv6 processing.
6. When a 6a44 client having a 6a44 address has an IPv6 packet to
send whose destination IS NOT in the same the same customer
site, it encapsulates the packet in a UDP/IPv4 packet whose
destination is UDP/IPv4 address of 6a44 relays. It then sends
the IPv6/UDP/IPv4 packet.
7. When a 6a44 relay receives via its IPv4 interface a valid IPv6/
UDP/IPv4 packet whose destination IS one of its 6a44 clients, it
forwards the contained IPv6 packet in a modified IPv6/UDP/IPv4
packet. The UDP/IPv4 destination of this packet is found in the
IPv6 destination address.
8. When a 6a44 client receives a valid IPv6/UDP/IPv4 packet from a
6a44 relay, it decapsulates the IPv6 packet and submits it to
further IPv6 processing.
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9. When a 6a44 relay receives via its IPv4 interface a valid IPv6/
UDP/IPv4 packet whose IPv6 destination IS NOT one of its 6a44
clients, it decapsulates the IPv6 packet and sends it via its
IPv6 interface.
10. When a 6a44 relay receives via its IPv6 interface a valid IPv6
packet whose destination is one of its 6a44 clients, it
encapsulates the packet in a UDP/IPv4 packet whose destination
is the UDP/IPv4 address found in the IPv6 destination address.
It then sends the resulting IPv6/UDP/IPv4 packet via its IPv4
interface.
11. To maintain the NAT44 mapping of its 6a44 tunnel, and to quickly
detect the need to change its 6a44 address in case of NAT44
reset, a 6a44 client sends from time to time a bubble to the
6a44 relay address (see Section 6.5.1) .
12. When a 6a44 relay receives via its IPv4 interface an IPv6/UDP/
IPv4 packet whose IPv6 and UDP/IPv4 source addresses are not
consistent, it discards the invalid packet, and returns a bubble
to the UDP/IPv4 source address. (This permits the 6a44 client
at this address to update its IPv6 address).
5. 6a44 Addresses
The 6a44 IPv6 address an ISP assigns to a host must contain all
pieces of information needed to reach it from other IPv6 addresses.
These piecesa are, as illustrated in Figure 3:
o the 6a44-network IPv6 prefix D (a /48 the ISP has assigned to its
6a44 relays);
o the customer-site IPv4 address N (either global IPv4 or, if the
ISP uses a [NAT444] model, private IPv4);
o the mapped port Z of the 6a44 tunnel (i.e. the external port
assigned by the NAT44 to the tunnel that the client maintains
between its UDP/IPv4 local address A:W and the 6a44-relay UDP/IPv4
address B:W).
o the client local IPv4 address A (i.e. the private IPv4 address
assigned to the client in its customer site; it is needed for
intra-site IPv6 connectivity).
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Internet-Draft Native IPv6 Behind NAT44 CPEs (6a44) April 20126. Specification of Clients and Relays6.1. Packet Formats6.2. IPv6 Packet Encapsulations
For NAT44 traversal, an IPv6 packet transmitted from a 6a44 client to
a 6a44 relay or conversely is encapsulated in a UDP/IP packet whose
source and destinations addresses are those of the two endpoints (A:W
and B:W in notations of Figure 3). The IPv4 packet is that of a
complete datagram (its more-fragment bit is set to 0, its offset is
set to 0, and its datagram identification may be set to 0). The UDP
checksum is set to 0 (there is no need for an additional layer of
checksum protection). The length of the IPv6 packet SHOULD NOT
exceed 1280 octets (see Section 6.4).
Octets: |0 |20 |28 |68 |
+----------+---+-------------------+-------//-----+
| IPv4 |UDP| IPv6 header | IPv6 payload |
+----------+---+-------------------+-------//-----+
An IPv6 packet transmitted from a 6a44 client to another 6a44 client
of the same site is encapsulated in an IPv4 packet whose source and
destination addresses are the private-IPv4 addresses of the two
hosts. The IPv4 packet is that of a complete datagram (its more-
fragment bit is set to 0, its offset is set to 0, and its datagram
identification may be set to 0). The size of the IPv6 packet SHOULD
NOT exceed 1280 octets for off-link destinations, and MUST NOT exceed
the link MTU minus 20 octets for on-link destinations (see
Section 6.4).
Octets: |0 |20 |60 |
+----------+-------------------+-------//-----+
| IPv4 | IPv6 header | IPv6 payload |
+----------+-------------------+-------//-----+
6.3. 6a44 Bubbles
A Bubble is a UDP/IPv4 packet whose UDP payload comprises a "6a44-
client IPv6 prefix" field and a "Bubble ID" field, and whose UDP
checksum is set to 0.
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"6a44-client IPv6 prefix" field
. from a 6a44 client = 0
. from a 6a44 relay = 6a44-client IPv6 prefix
|
Octets: |0 |20 |28| |40 |48
+----------+---+--|-+---+
| IPv4 |UDP| . | . |
+----------+---+----+-|-+
|
"Bubble ID" field
. from a 6a44 client: a client-selected value
. from a 6a44 relay:
- in a response bubble, copy of the received bubble ID
- in an error signaling bubble, 0
6a44 BUBBLE FORMAT
Figure 4
In a bubble from a 6a44 client to a 6a44 relay, the "6a44-client IPv6
prefix" field is only reserved space for the response. It is set to
0. In a bubble from a 6a44 relay to a 6a44 client, it contains the
IPv6 prefix of the client.
In a bubble from a 6a44 client to a 6a44 relay, the "Bubble ID" field
contains a randomly chosen value, renewed in circumstances defined in
Section 6.5.1. In a bubble from a 6a44 relay to a 6a44 client: if
the bubble is a response to a bubble received from the client, the
field contains the value found in the received bubble; if the bubble
is it is a reaction to a received IPv6/UDP/IPv4 packet whose IPv6 and
UDP/IPv4 sources are inconsistent, the field is set to 0. The
purpose of this field is a protection against 6a44-relay spoofing
attacks (see Section 7).
In order to preserve forward compatibility with any extension of
bubble formats, should one prove useful in the future, 6a44 clients
and 6a44 relays MUST accept to receive bubbles whose UDP payloads
lengths are longer than 20 octets (up to that of an IPv6-packet
header since, as detailed in Section 6.6.2 and Section 6.5.3, bubbles
are recognized by their lengths being shorter than that of tunneled
IPv6 packets).
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Internet-Draft Native IPv6 Behind NAT44 CPEs (6a44) April 20126.4. Maximum Transmission Units
Reassembly of a fragmented IPv4 datagram necessitates to remember its
identifier from reception of the first fragment to reception the last
one, and necessitates a timeout protection against packet losses. If
such an IP-layer stateful processing would be necessary for 6a44, it
would make it more complex than needed, would introduce a
vulnerability to denial of service attacks, and would impose that all
fragments of a fragmented IPv4 datagram go to the same relay. This
last point would be a constraint on how load balancing may be
performed between multiple 6a44 relays, and would therefore be
detrimental to scalability.
For 6a44 processing to remain completely stateless, IPv4 packets
containing encapsulated IPv6 packets must never be fragmented. For
this:
o In customer sites, 6a44 clients MUST have IPv4 link MTU's that
support encapsulated IPv6 packets of lengths up to 1280 octets,
i.e., for IPv6/UDP/IPv4 packets that traverse the CPE, link MTU's
of at least 1280+20+8=1308 octets. (This condition is in general
satisfied).
o For the same reason, 6a44 ISP networks must have IPv4 path MTU's
of at least 1308 octets. (This condition is in general
satisfied).
o 6a44 clients SHOULD limit the size of IPv6 packets they transmit
to off-link destinations to 1280 octets. They MUST also limit the
size of IPv6 packets they transmit to on-link destinations to the
link MTU minus the 20 octets of an IPv4 encapsulation header.
o 6a44 relays SHOULD set their IPv6 MTU to 1280. (If a relay
receives an IPv6 packets longer than this MTU via its IPv6
upstream interface, it MUST return ICMPv6 Packet Too Big message).
Typical ISP networks have a path MTU's that would permit IPv6
MTU's of 6a44 to be longer than 1280, but taking 1280 octets is a
precaution that guarantees against problems with customer sites
that may have internal path MTU's smaller than those supported by
their ISP networks.
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For this, the 6a44 client SHOULD apply the equivalent of the
following TM-x rules, illustrated in Figure 5:
TM-1 At initialization, a timer value T1 is randomly chosen in the
recommended range 1 to 1.5 seconds, and the "6a44 disabled"
state is entered. (Randomness of this value is a precaution to
avoid that, if many hosts happen to be re-initialized at the
same time, the bubble traffic resulting from the following
rules be synchronized.)
TM-2 In the "6a44-disabled" state, if it appears that the the
interface has no IPv6 native address BUT has a private IPv4
address, then: the Attempt count (a local variable) is set to
1; a new Bubble ID (another local variable) is randomly chosen;
a bubble is sent with this bubble ID; the "Bubble sent" state
is entered with the timer set to T1.
TM-3 In the "Bubble sent" state, if the timer expires AND the
Attempt count is less than 4, then: the Attempt count is
increased by 1; a new bubble is sent with the current Bubble
ID; the "bubble sent" state is re-entered with the timer reset
to T1.
TM-4 In the "Bubble sent" state, if a bubble is received, then: the
6a44-client IPv6 address is set to the received 6a44-client
IPv6 prefix followed by the host local IPv4 address; the
"Bubble received" state is entered with the timer set to T2
whose recommended value is 30 seconds minus 4 times T1.
TM-5 In the "Bubble sent" state, if timer T1 expires AND the Attempt
count is equal to 4, then: the "No 6a44 relay" state is entered
with the timer set to T3 whose recommended value is 30 minutes.
TM-6 In the "Bubble sent" state, OR the "Bubble received" state, OR
the "No 6a44 relay" state, if a IPv6 native address is obtained
by some other mean, OR if the private IPv4 address of the host
is no longer valid, then: the timer is disarmed; the "6a44
disabled" state is entered.
TM-7 In the "Bubble received" state, if the timer T2 expires, then:
the Attempt count is reset to 1; a new Bubble ID is randomly
chosen; a bubble is sent with this bubble ID; the "Bubble sent"
state is entered with the timer set to T1.
TM-8 In the "Bubble received" state, if a bubble is received, then:
the timer is reset to T2. (NOTE: Since a bubble is received by
a 6a44 client either in response to a bubble it has sent or in
reaction to a packet it has sent with inconsistent IPv6 and
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UDP/IPv4 source addresses, receiving a bubble is a sign that
the tunnel mapping reported in the received bubble prefix has
recently been used in BOTH directions, a condition required by
some NAT44s to maintain their mappings).
TM-9 In the "no 6a44 relay" state, if the timer expires, then: the
Attempt count is reset to 1; a new Bubble ID is randomly
chosen; a bubble is sent with this bubble ID; the "Bubble sent"
state is entered with the timer set to T1.
6.5.2. Client Transmission
An 6a44 client transmits packets according to the following CT-x
rules. In figures which illustrate these rules, symbols of Section 5
are re-used; packets are represented as a succession of significant
fields separated by commas, with sources preceding destinations as
usual; != means different from.
CT-1 BUBBLE SENT BY A 6a44 CLIENT
(IPv4, A, B, UDP[::/96, <current Bubble ID>])
|
+-------+--------+ |
| | 6a44 | |
| | client +------>---------- >B:W
| |function|A:W< UDP/IPv4
+-------+--------+
Host
Bubbles are transmitted from time to time. Conditions of their
transmission are specified specified in Section 6.5.1, and
their format is specified in Section 6.3.
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CT-2 IPv6/IPv4 PACKET SENT TO A HOST OF THE SAME SITE
[IPv6, <D.N.Z.A>, <D.N..A2>,...]
|
| (IPv4, A, A2, IP-in-IP[encapsulated packet])
| |
+----|--+--------+ |
| | | 6a44 | |
| -->--+ client +------>------ >A2
| IPv6 |function|<A IPv4
+-------+--------+
Host
If an IPv6 packet is submitted for transmission with ALL the
following conditions satisfied, the 6a44 client MUST
encapsulate the IPv6 packet in an IPv4 packet whose protocol is
set to IP in IP (protocol = 41), and whose IPv4 destination is
copied from the last 32 bits of the IPv6 destination: (1) the
IPv6 source address is the 6a44-client IPv6 address; (2) the
IPv6 destination is a 6a44 address of the same site (it has the
same 80 bits as the 6a44-client IPv6 address); (3) either the
IPv6 packet does not exceed 1280 octets, or it is longer but it
does not exceed the IPv4 link MTU minus 20 octets and the IPv4
destination address starts with the IPv4 link prefix.
CT-3 IPv6/UDP/IPv4 PACKET TO A HOST OF ANOTHER SITE
[IPv6, <D.N.Z.A>, X != <D.N...>, ...]
|
| (IPv4, B, A, UDP(W, W, [encapsulated packet])
| |
+----|--+--------+ |
| | | 6a44 | |
| -->--+ client +------>---------- >B:W
| IPv6 |function|A:W< UDP/IPv4
+-------+--------+
Host
If an IPv6 packet is submitted for transmission and ALL the
following conditions are satisfied, the IPv6 packet MUST be
encapsulated in a UDP/IPv4 packet whose destination is the
6a44-relay anycast address, and whose source and destination
ports are both the 6a44 port: (1) the source address is the
local 6a44-client IPV6 address; (2) The destination is not a
6a44 address of the same site (its first 80 bits differ from
those of the 6a44-client IPv6 address); (3) The IPv6 packet
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does not exceed 1280 octets.
CT-4 IPv6 PACKET THAT DOESN'T CONCERN 6a44
If an IPv6 packet is submitted to the 6a44 client function for
transmission with an IPv6 source address that is not the 6a44-
client IPv6 address, the packet does not concern 6a44. It MUST
be left for any other IPv6 transmission function that may apply
(the source address can be a link-local address or a ULA of
[RFC4193]).
6.5.3. Client Reception
Upon reception of an IPv4 packet, a 6a44 client applies the following
CR-x rules:
CR-1 BUBBLE RECEIVED FROM A 6a44 RELAY
(IPv4, B, A, UDP(w, w, [<D.N.Z>, <current bubble ID>])
|
+-------+--------+ |
| | 6a44 | |
| | client +------<---------- <B:W
| | |A:W< UDP/IPv4
+-------+--------+
Host
(updates D.N.Z)
If ALL the the following conditions are satisfied (i.e. the
packet is a 6a44 bubble from a 6a44 relay), the 6a44-client
IPv6 address MUST be updated using the received IPv6 prefix
D.N.Z: (1) the IPv4 packet contains a complete UDP datagram
(protocol = 17, offset = 0, more-fragment bit = 0); (2) Both
ports of the UDP datagram are the 6a44 port, and the payload
length is enough to contain a 6a44-client IPv6 prefix and a
Bubble ID but shorter than an IPv6-packet header(protocol = 17,
UDP payload length = at least 20 octets and less than 40
octets); the received Bubble ID matches the current value of
the Bubble-ID local variable.
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CR-2 IPv6/IPv6 PACKET FROM A HOST OF THE SAME SITE
(IPv4, A2, A, IP-in-IP, [IPv6, <D.N..A2>, <D.N.Z.A>, ...])
|
[decapsulated packet] |
| |
+----|--+--------+ |
| | | 6a44 | |
| --<--+ client +------<------ <A2
| IPv6 | |A< IPv4
+-------+--------+
Host
If ALL the following conditions are satisfied (i.e. the packet
comes from a 6a44 client of the same site), the 6a44 client
MUST decapsulate the inner packet and treat it as a received
IPv6 packet: (1) the IPv4 packet contains a complete UDP
datagram (protocol = 17, offset = 0, more-fragment bit = 0);
(2) both ports of the UDP datagram are the 6a44 port, and the
UDP payload is an IPv6 packet (UDP length of at least 40
octets, version = 6); (3) the IPv6 source address is one of the
same site (the first 80 bits match those of the 6a44-client
IPv6 address; (4) its last 32 bits are equal to the IPv4 source
address; (5) the IPv6 destination address is the 6a44-client
IPv6 address.
CR-3 IPv6/UDP/IPv4 PACKET FROM A HOST OF ANOTHER SITE
(IPv4, B, A, UDP(W, W, [IPv6, X, <D.N.Z.A>,...])
|
[decapsulated packet] |
| |
+----|--+--------+ |
| | | 6a44 | |
| --<--+ client +------<---------- <B:W
| IPv6 | |A:W< UDP/IPv4
+-------+--------+
Host
If ALL the following conditions are satisfied (i.e. the packet
has been relayed by a 6a44 relay), the 6a44 client MUST
decapsulate the inner packet and treat it as a received IPv6
packet: (1) the IPv4 packet contains a complete UDP datagram
(protocol = 17, offset = 0, more-fragment bit = 0); (2) the UDP
payload is an IPv6 packet (length of at least 40 octets,
version = 6); (3) the UDP/IPv4 source address is the 6a44-relay
UDP/IPv4 address; (4) the IPv6 destination address is the 6a44-
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client IPv6 address.
CR-4 RECEIVED IPv4 PACKET OTHER THAN 6a44
If ANY of the following conditions is verified, the received
IPv4 packet does not concern 6a44 and MUST therefore be left
for any other IPv4 reception function that may apply: (1) the
IPv4 payload is neither UDP nor IPv6 (protocol = neither 17 nor
41, or protocol = 41 and IP version in the payload is not = 6);
(2) the IPv4 packet is an IP-datagram fragment other than the
first one (offset > 0); (3) the IPv4 packet contains the first
or unique fragment of a UDP datagram (protocol = 17, offset =
0), with neither port equal to the 6a44 port.
6.6. 6a44 Relay Specification6.6.1. Relay Reception in IPv6
Upon reception of a packet via its IPv6 interface with a destination
address starting with the 6a44-network IPv6 prefix, a 6a44 relay MUST
apply the following RR6-x rules:
RR6-1 VALID IPv6 PACKET FROM OUTSIDE THE 6a44 ISP NETWORK
[IPv6, (X != <D...> AND != <Teredo(IPv4=B)>), <D.<N != B>.Z...>,...]
|
(IPv4, B, N, UDP(W, Z, |
[encapsulated packet])) |
| |
| +--------+ |
| >B:W | 6a44 |D/48< |
N:Z< ---<--------| relay |-------<---- D.N.Z...<
IPv4 | | IPv6
+--------+
If ALL the following conditions are satisfied, the IPv6 packet
MUST be encapsulated in an UDP/IPv4 packet whose UDP/IPv4
destination is copied from bits 48 to 95 of the IPv6
destination address: (1) the IPv6 source address is not that
of a 6a44 client of the ISP (it does not start with the 6a44-
network IPv6 prefix); (2) the IPv6 source address is not a
Teredo address whose embedded UDP/IPv4 address is the 6a44-
relay anycast address; (3) the customer-site IPv4 address
embedded in the 6a44 destination address is not the 6a44-relay
anycast address; (4) the packet has at most 1280 octets.
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RR6-2 INVALID IPv6 PACKET FROM OUTSIDE THE 6a44 ISP NETWORK
If ANY of the following conditions is satisfied, the IPv6
packet MUST be discarded : (1) the packet has more than 1280
octets (in this case, an ICMP Packet Too Big error message
MUST be returned to the source); (2) the customer-site IPv4
address embedded in the IPv6 destination address is the 6a44-
relay anycast address; (3) the IPv6 source address is a Teredo
address whose embedded IPv4 address is the 6a44-relay anycast
address.
6.6.2. Relay Reception in IPv4
Upon reception via its IPv4 downstream interface of an IPv4 packet
that contains a complete IP datagram (fragment offset = 0 and more-
fragment bit = 0), and that contain a UDP datagram whose UDP/IPv4
destination is the 6a44-relay UDP/IPv4 address, a 6a44 relay MUST
apply the following rules:
RR4-1 BUBBLE FROM 6a44 CLIENT
(IPv4, N, B, UDP(Z, W, [::/96, bubble ID]))
|
IPv4 | +--------+
>B:W ------->----| |
>B:W| 6a44 |
| relay |
N:Z< -------<----| |
IPv4 | +--------+
|
|
(IPv4, B, N, UDP(B, W, [<D.N.Z>, bubble ID]))
If the following condition is satisfied, the 6a 44 relay MUST
return to the source a bubble derived from the received one by
permuting its UDP/IPv4 source and destination, and by putting
in its 6a44-client-IPv6-prefix field the received UDP/IPv4
source address: the UDP payload is a bubble, i.e has at least
20 octets and less than 40 octets
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the 6a44-network IPv6 prefix, followed by the UDP/IPv4 source
address of the received packet; (4) the IPv6 destination
address does not start with the 6a44-network IPv6 prefix and
is not a Teredo address whose embedded IPv4 address is the
6a44-relay anycast address.
RR4-4 INVALID IPv6/UDP/IPv4 PACKET
If ANY other case, the 6a44 relay MUST discard the packet.
6.7. Implementation of Automatic Sunset
6a44 is designed as an interim transition mechanism, not to be used
any longer than strictly necessary. Its sole purpose is to
accelerate availability of IPv6 native addresses where, for any
reason, CPE's cannot quickly be replaced, or where, for any reason,
ISP networks cannot quickly support dual-stack routing or 6rd.
A 6a44-capable ISP can first have an increase of its 6a44 traffic, as
more and more hosts behind IPv4-only CPEs support the 6a44 client
function. But it should later have a decrease of this traffic as
more and more CPE's operate in dual stack.
When this traffic becomes sufficiently negligible, it may, after due
prior notice, discontinue 6a44-relay operation. This terminates its
sunset procedure.
In a host that obtains a IPv6 native address by some other mean than
6a44, the effect of having the 6a44 function in its protocol stack is
inexistent. OS providers may therefore keep this function in their
code for many years. When it becomes clear that the number of users
of this unction has become negligible they can delete it from later
releases. This terminates their sunset procedure.
7. Security Considerations
Incoming reachability:
Hosts that acquire 6a44 addresses become reachable from the
Internet in IPv6 while they remain unreachable in IPv4 at their
private IPv4 addresses.
For ordinary use, this should not introduce a perceptible new
security risk for two reasons: (1) hosts can, without IPv6, use
NAT44 hole-punching techniques such as ICE of [RFC5245]) to
receive incoming connections; (2) modern operating systems that
support IPv6 have by default their own protections against
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incoming connections.
If nevertheless 6a44 reachability across an ordinary NAT44 has to
be barred, this can be done by configuring its port-forwarding
function with the 6a44 port bound to any internal address that is
not assigned to any host. Thus, no bubble from a 6a44 relay can
reach any 6a44-capable host, and this is sufficient to prevent
hosts from using 6a44.
For more sophisticated uses with managed firewalls, default
configuration are in general such that packets that are not
explicitly authorized are discarded. Thus, 6a44 can be used only
if the 6a44 port is consciously opened to incoming traffic.
Subscriber authentication:
Any authentication that applies to an IPv4 address extends its
effect to 6a44 addresses that are derived from it.
Host-address spoofing:
With ingress filtering required in 6a44 ISP networks, and with
address checks of Section 6, no new IPv6 address-spoofing
vulnerability is introduced by 6a44.
Address-and-port scanning:
To mitigate the (limited) risk of a malicious user trying to scan
address-and-port IPv4 couples to reach a host, Teredo addresses
contain 12 random bits [RFC5991]. 6a44 addresses have no random
bits but contain local IPv4 addresses of clients. Since possible
values of these addresses are not deterministically known from
outside customer sites, and are in ranges that can be configured
in typical NAT44s, some protection against address and port
scanning is thus achieved. This protection may be less effective
than that achieved with random bits, but is in any case better for
6a44 IPv6 addresses than for IPv4 addresses alone.
Denial-of-service:
Provided 6a44 relays are provisioned with enough processing power,
which is facilitated by their being completely stateless, 6a44
introduces no denial of service vulnerabilities of its own.
Routing-loops:
A risk of routing-loop attacks has been identified in
[draft-ietf-v6ops-tunnel-loops]. Without precaution, it applies
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to some combinations of automatic-tunnel mechanisms such as 6to4,
ISATAP, 6rd and Teredo. This risk does not exist with 6a44 for
the following reasons:
1. When an packet enters a 6a44 relay via its IPv6 interface:
+ An IPv6/UDP/IPv4 packet cannot be sent to another 6a44
relay because its IPv4 destination would have to be 6a44-
relay IPv4 address. This is prevented by rule RR6-1 of
Section 6.6.1.
+ If an IPv6/UDP/IPv4 packet is sent to the address of a 6to4
relay, 6rd relay, or ISATAP relay, it will be discarded
there because these relays don't accept UDP/IPv4 packets.
+ If an IPv6/UDP/IPv4 packet is sent to a Teredo relay, it
will be discarded there because: (1) Teredo relays check
that the IPv4 addresses that is embedded in the IPv6 source
address of a received IPv6/IPv4 packet does match the IPv4
source address of the encapsulating packet (section 5.4.2
of [RFC4380]); (2) encapsulating packets sent by 6a44
relays have the 6a44-relay anycast address as IPv4 source
address; (3) a 6a44 relay forwards a received IPv6 packet
as an IPv6/UDP/IPv4 packets only if its IPv6 source address
is not a Teredo address whose embedded IPv4 address is the
6a44-relay IPv4 address.
2. When a packet enters a 6a44 relay via its IPv4 interface:
+ The received packet cannot come from another 6a44 relay (as
just explained, 6rd relays do not send IPv6/UDP/IPv4
packets to other 6a44-relays).
+ If the IPv4 packet comes a 6to4 relay, a 6rd relay, or an
ISATAP relay, its IPv6 encapsulated packet cannot be
forwarded (the received packet is IPv6/IPv4 instead of
being IPv6/UDP/IPv4, as required by rules RR4-2 and RR4-3
of Section 6.6.2).
+ If the received packet is an IPv6/UDP/IPv4 packet coming
from a Teredo relay, this packet cannot have been sent to
the Teredo relay by a 6a44 relay ((1) in order to reach the
6a44 relay, the IPv6 destination of the IPv6 encapsulated
packet must be a Teredo address whose embedded IPv4 address
is the 6a44-relay anycast address (section 5.4.1 of
[RFC4380]); (2) a 6a44 relay does not forward via its IPv6
interface an IPv6 packet whose destination is a Teredo
address whose embedded IPv4 address is the 6a44-relay
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anycast address (rule RR4-3 of Section 6.6.2).
6a44-relay spoofing:
In a 6a44 network, no node can spoof a 6a44 relay because ingress
filtering prevents any 6a44-relay anycast address to be spoofed.
In a network that does not support ingress filtering (and
therefore is not a 6a44 network):
* 6a44 packets sent by 6a44-capable hosts are discarded in the
IPv4 backbone because their IPv4 destination, the 6a44-relay
anycast address, does not start with any ISP assigned prefix.
* If an attacker tries to send to a 6a44-capable host a faked
relay-to-client bubble, the probability that it would be
accepted by its destination is negligible. It would require
that all the following conditions be simultaneously satisfied:
+ The UDP/IPv4 destination set by the attacker must reach a
NAT44 node in which it is the external mapping of a 6a44
tunnel established by a 6a44-capable host.
+ This host must be in the "Bubble sent" state, the only one
in which it listens to bubbles when its ISP is not 6a44
capable. This state is taken only for a few seconds every
30 minutes (rule TM-5 of Section 6.5.1).
+ This host accepts the bubble only if its bubble ID has the
right value, an extremely unlikely possibility with a 64-
bits randomly chosen Bubble ID (see Section 6.3).
If, despite its negligible probability, a host would accept a
faked bubble, the effect would be that a 6a44-capable host
believes, for about 30 seconds, that it has an IPv6 address.
All IPv6 packets it would then send with this address as source
would not reach any destination (no relay will receive them,
and and no host of the same site will accept them). The
consequence would therefore not impair security.
8. IANA considerations
IANA is solicited:
to assign 192.88.99.2 as the 6a44 IPv4 anycast address;
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to assign a well known UDP port as the 6a44 well known port.
Proposed value is the currently unused 1009.
The choice of 192.88.99.2 as 6a44 IPv4 anycast address doesn't
conflict with any existing IETF specification because:
o It starts with the 6to4 prefix 192.88.99.0/24 of [RFC3068].
o It differs from the only currently assigned address that starts
with this prefix (the anycast address of 6to4 relays 192.88.99.1
of [RFC3068]).
This choice is made to permit implementations of 6a44 relays both in
physical nodes that are independent from any 6to4 relay or, if found
more optimum, in nodes in which 6to4 relays and 6a44 relays are
collocated.
9. Acknowledgments
This specification, whose origin is a convergence effort based on two
independent proposals, [6rd+] and [SAMPLE], has benefited from
various suggestions. Comments have been received during this
process, in particular from Dave Thaler, Fred Templin, Ole Troan,
Olivier Vautrin, Pascal Thubert, Washam Fan, and Yu Lee. Authors wish
to thank them, and all others, for their useful contributions.
Special recognition is due to Dave Thaler whose detailed review led
to a few useful modifications.
10. References10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
10.2. Informative References
[6rd+] Despres, R., "Rapid Deployment of Native IPv6 Behind IPv4
NATs (6rd+) - draft-despres-softwire-6rdplus-00",
July 2010.
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